1. Field of the Invention
The present invention relates to a capacitor comprising a metal-deposited polyester film. More particularly, the present invention relates to a capacitor which comprises a polyester base film and a metal layer deposited thereon with improved adhesion therebetween and is excellent in moist-heat resistance.
2. Description of the Related Art
A polyester film, typically a polyethylene terephthalate film is widely used as a base film of a capacitor since it is excellent in mechanical properties, heat resistance and electrical properties. With the recent progresses in various electronic equipments, the properties of the polyester film have been improved. One of the properties of the polyester film to be improved is a long-term stability against moist heat. That is, a metal-deposited polyester film has a drawback that the adhesion between the base film and the deposited film layer, in particular, the adhesion in a moist heat atmosphere, namely the moist-heat resistant adhesion is insufficient. Though a conventional capacitor is sheathed by an epoxy resin having a sufficient thickness, when it is stored at high temperature and high humidity for a long time, moisture penetrates in an interface between the base film and the deposited metal layer so that an electrostatic capacity of the capacitor greatly decreases due to corrosion of vaporized electrodes. Then, it is highly required to improve the moist-heat resistance of the capacitor in view of the long-term stability.
To this end, it is proposed to coat a low viscosity epoxy resin under reduced pressure instead of atmospheric pressure to penetrate the resin deep into the capacitor using a difference between the inside pressure and the atmospheric pressure. However, the low viscosity epoxy resin foams under reduced pressure and adheres to lead wires, which causes a serious trouble that a solder is hardly applied on the lead wires when the capacitor is soldered to a printed-wiring board. In addition, the capacitor coated by the low viscosity epoxy resin has a poor self-healing property when an excess voltage is applied, so that a withstand voltage of the capacitor is decreased.
As a capacitor having good moist-heat resistance, Japanese Patent Publication Nos. 59612/1990 and 59613/1990 disclose a film capacitor having a coating layer of polyvinylidene chloride, and a film capacitor having a coating layer comprising a melamine resin and/or a urea resin, respectively. However, with the resins disclosed in these Patent Publications, the properties of the capacitor are not necessarily maintained in a high humidity high temperature atmosphere. For example, while the decrease of electrostatic capacity of the capacitor can be prevented at 40.degree. C. in a moist-heat atmosphere, the electrostatic capacity of the capacitor quickly decreases at a temperature of 60.degree. C. or higher in the moist-heat atmosphere.
Since the electronic equipments are remarkably improved recently, demands on the long-term stability, in particular the long-term moist-heat stability of the capacitor properties are still increasing.